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Hallucigenia

Hallucigenia is an animal found as fossils in the Middle Cambrian-aged Burgess Shale formation of British Columbia, Canada (H. sparsa) and the Lower Cambrian Maotianshan shale of China (H. fortis). The genus was named by Simon Conway Morris when he examined specimens of Charles Walcott's Burgess Shale worm genus 'Canadia' in 1979. Morris found that the 'genus' included several different animals. One of them was not a polychaete worm; Morris named it Hallucigenia sparsa due to its "bizarre and dream-like quality." (like a hallucination). Stephen Jay Gould thought that Hallucigenia was unrelated to any living species (6), but most palaeontologists believe it was a relative of modern arthropods (3,4).

Hallucigneia was a 0.5-3 cm-long, long, narrow, worm-like animal is wormlike with a poorly defined blob, or stain, on one end. This "blob" was designated the 'head,' but lacked a mouth, eyes or other sensory organs. Morris originally interpreted the animal with seven pincer-tipped tentacles lined up on one side and seven pairs of jointed spines on the other. Six of the tentacles were paired with spines, with one in front of the spines. There were six smaller tentacles, which may be configured in three pairs behind the seven larger ones. The body continued with a flexible, tube-like, tail-like extension behind the tentacles. Morris noted that the animal had no obvious head and neither of the two types of appendages seemed appropriate for any reasonable form of locomotion. He assigned the blob as the head, the spines as legs and the tentacles as feeding appendages. Morris demonstrated a workable, if improbable, method of walking on the spines. Only the forward tentacles could easily reach to the 'head', so a mouth on the head would have to be fed by passing food along the line of tentacles. Morris suggested that a hollow tube in each tentacle might be a mouth. This raised questions as to how it would walk on the stiff legs, but it was accepted for a time (1,6). Some people though Hallucigenia was an appendage of a larger, unknown animal (6). Given its uncertain taxonomy, Hallucigenia was tentatively placed within the phylum Lobopodia, a catch-all clade of "worms with legs." In 1991, Lars Ramskold and Hou Xianguang worked with the "hallucigenid," Microdictyon, from the lower Cambrian Maotianshan shales of China, and reinterpreted Hallucigenia as an Onychophore (velvet worm). They inverted it, interpreting the tentacles, which they believe to be paired, as walking structures and the spines as protective (2). None of the Burgess Shale specimens, not their Chinese counterparts, show any sign of pairing in the large tentacles. The pairing is based on a dissection of the fossil, probably revealing a second tentacle structure. Ramskold and Hou believe the blob-like 'head' is a stain, not a preserved portion of the anatomy. Unlike its contemporary Aysheaia, Hallucigenia has very little resemblance to modern Onychophora. The elongated, clawed legs bear little resemblance to the paired annulated legs of the Onychophora. It is unknown what the spines were made of and how much 'protection' they offered. They do not seem to be preserved independent of the soft-shelled animals as carbonate or chitinous shells would probably be. The specimens are hypothesized to have seven pairs of long, flexible legs, but do not show any examples of paired legs. Some paleontologists believe that Hallucigenia may be an "armoured lobopod" related to Anomalocaris. It could also being related to the Onychophora, coming from a time during or near the split of the two closely related groups (5). In 2002, Desmond Collins suggested that Hallucigenia fossils showed male and female forms, one with a rigid trunk, robust neck and globular head and a thinner form with a small head (3).

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